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UCC2808A-1: In soft start,Change UCC2808A Pin 1(COMP) PWM from 0% to 50%,The expected comp voltage is from 0V to 5V.

Part Number: UCC2808A-1
Other Parts Discussed in Thread: UC2843,

Dear Guys:

              In soft start,Change UCC2808A  Pin 1(COMP) PWM from 0% to 50%,The expected comp voltage is from 0V  to 5V.

However, it is actually increased to about 1.5v, and this voltage will not continue rise。The customer selected UC2843 test comp pin did not find this problem!

Can you tell me why? Is there any solution


  • Hello ,

    It is acceptable that the COMP value can be different in both of these controllers.
    If your power supply is operating correctly then both of these controllers should provide the correct output voltage.

    Does the UCC208A provide the correct output voltage ?



  • Hi John:

               UCC2808A-1  Can provide the correct output Voltage。Change duty cycle, this comp foot voltage does not change very much.  meanwhile,Peak current is high at soft start。That's why?Is there a solution

  • Hi Tony,

    I think what you are seeing is that the transfer function gain is different for both of these parts.

    It is to be expected that the COMP value will not be the same if the loop gain is different for the UCC2808A and the UC2843.

    The loop feedback will ensure that the output voltage is correct but there is no reason that both COMP values will be the same.

    Also the UCC2808A has a fixed internal soft start and the UC2843 may have an adjustable soft start depending on your schematic design.

    I think you should decide on a part you wish to use and design your power supply around the part.



  • Hi  John;

        Whether there is no reference design for loop gain? Thank you 。



  • Hi Tony,

    Sorry we do not have this information in a reference design.

    I will close this post since I think there is no further information available.

    Please open a new post if you need any further help.